1. Field of the Invention
The present invention relates to an electric device and a method of driving the same. Particularly, the present invention relates to a measuring instrument for fluids such as gases, liquids, and fog-like fluids (including atomized fluids and gases and fluids containing solid powder) and to a method of operating the measuring instrument. More particularly, the invention relates to a measuring instrument for measuring flow rates or flow velocities of fluids, discriminating between fluids, measuring the ratio of one constituent of a mixed fluid to the other, and measuring the concentration (e.g., humidity) of a substance contained in a fluid. Also, the invention relates to a method of operating this measuring instrument.
2. Prior Art
A known instrument for measuring a flow rate makes use of a thermistor. In particular, a fluid takes heat away, thus lowering the temperature of the thermistor. This instrument makes use of this phenomenon. Generally, when the thermistor is in contact with the fluid, the amount of heat taken away from the thermistor depends on the flow rate and so the output from the thermistor has a certain correlation with the flow rate. Utilizing this correlation, the flow rate can be calculated from the output from the thermistor.
A flow rate is the product of the cross-sectional area of a fluid and the flow velocity. It is assumed that a fluid flows through a circular pipe having an inside diameter r at a flow velocity of v. The flow rate is given by vxcfx80r2. Accordingly, we will concentrate on the flow rate in the description made below. If the cross-sectional area of the fluid is known, then the flow rate and the flow velocity can be found simultaneously.
Generally, a thermistor refers to a semiconductor having a negative temperature coefficient of resistance. Instead of such a thermistor, a metal such as platinum having a positive temperature coefficient of resistance can be used. That is, any material can be used as long as its resistance changes with temperature. Devices using a material whose resistance changes with temperature are collectively known as thermally sensitive resistors, temperature sensing elements, or resistance thermometers. Thermally sensitive resistors referred to herein mean materials whose resistances change with temperature.
Another structure uses a heating resistor exposed to a fluid. The amount of heat taken away from this resistor depends on the flow rate. This structure makes use of this phenomenon. In this system, the flow rate can be calculated by measuring the electrical current flowing through the heating resistor.
A further structure employs a heating element in contact with a fluid. Heat is taken away from the heating element by the fluid. A temperature sensing element (e.g., a platinum sensor) disposed independent of the heating element measures the amount of heat carried away by the fluid. The flow rate is calculated from this amount of heat.
In these methods, a higher sensitivity is effectively obtained by increasing the amount of heat taken away by the fluid. In order to enhance the response speed, it is necessary to reduce the heat capacity of the temperature sensing element as small as possible.
The flowmeters constructed as described above have the problem that only a narrow range of flow rates can be measured. That is, their dynamic ranges are narrow. Specifically, these flowmeters are capable of precisely measuring flow rates within ranges only from 20 to 300 sccm or from 200 to 2000 sccm.
It is considered that these problems are chiefly due to the following reasons.
(1) Since the temperature sensing element is in a thermally quite unstable state, the linearity of the response to the heat is poor. Hence, the flowmeter cannot respond to thermal changes occurring in a wide range.
(2) In association with (1) above, heating is especially difficult to perform. It is impossible to do effective heating over a wide range of flow rates.
(3) If the heat capacity of the temperature sensing element is reduced to increase the response speed, it is impossible to treat large amounts of heat.
The item (1) described above occurs because it is difficult to build a structure which permits the fluid to effectively take heat away from the temperature sensing element over a wide range of flow rates and which, at the same time, effectively supplies heat to the sensing element.
The item (3) described above means that increasing the response speed conflicts with increasing the dynamic range, or the range of flow rate of the investigated fluid.
Accordingly, the present situation is that the prior art flowmeter is designed to measure a narrowed range of flow rates so that the heat amount treated by the temperature sensing element may not vary greatly. More specifically, the measured range of flow rates is narrowed. Within this range, the heat capacity of the temperature sensing element, the positional relation between the temperature sensing element and the heating element, their thermal relationship, and the electrical current flowing through the temperature sensing element are subtly adjusted to obtain the required sensitivity and measuring accuracy.
A further known structure is designed so that the heat amount supplied to a heating element changes according to the flow rate. The amount of heat taken away from a temperature sensing element is in proportion to the flow rate, irrespective of the flow rate value.
Temperature sensing elements sensitively detect temperature variations (i.e., temperature variations in the fluid) in the environment as well as flow rates. Therefore, where the temperature sensing elements are used in an environment where temperature varies, a problem takes place. Various proposals have been made to solve this problem. In actual usage, however, the measured flow rate is affected greatly by the fluid temperature.
Where a temperature sensing element or a heating element is exposed to a fluid, the material of the sensing element or heating element is corroded, depending on the kind of the fluid. As a result, the electrical and thermal characteristics are varied. To solve this problem, a method of coating the surface with a protective film may be contemplated. However, where the thermistor surface is coated with a protective film, the speed of the response to heat deteriorates. Also, the measuring accuracy is deteriorated by refraction of heat. Hence, this scheme is undesirable.
Flowmeters using thin diamond films are described in Sensors and Materials, 2, 6 (1991), pp. 329-346 and in Applications of Diamond Films and Related Materials, Y. Tzeng, M. Murakawa, A. Feldmand (Editors), Elsevier Science B. V., 1991, pp. 311-318. These two flowmeters are essentially the same in structure. In these flowmeters, a thin film of diamond is formed on a silicon substrate by CVD. Resistors formed inside the silicon substrate are in contact with two opposite ends, respectively, of the thin film. One of the resistors acts as a heating element, while the other serves as a temperature sensing element, or a sensor. When heat is conveyed from one end to the other end of the diamond film in the direction of the plane of the film, the fluid carries away heat from the surface of the thin film. This amount of heat is detected by the temperature sensing element mounted at the other end. However, these flowmeters do not have any characteristics which deserve attention.
Accordingly, in the present invention, a thermistor is formed on the surface of a thin film of diamond. This film is operated as a heat storage layer for supplying heat to the thermistor. A thermal equilibrium state which quickly satisfies Eq. (8) if a flow rate change occurs is realized. The temperature of the thermistor is settled in such a way that the flow rate of the fluid is constantly reflected.
W=K(Txe2x88x92T0)+G(Txe2x88x92T0)xe2x80x83xe2x80x83(8)
A first invention of the present application lies in a flow detector using a thin film of diamond which has a layer acting as a thermistor on its surface.
In the above-described first invention, the thin diamond film is preferably fabricated by chemical vapor deposition (CVD). No limitations are imposed on the conductivity type of the diamond film. Any thin diamond film to which one conductivity type is imparted by an appropriate doping method may be used. The layer acting as a thermistor can be fabricated from platinum or an alloy of platinum and other material by sputtering. Also, this layer may be made of a material generally capable of acting as a thermistor. The layer acting as a thermistor can be formed on the surface of the diamond film by doping the thin film with an impurity that imparts one conductivity type to the thin film. This thermistor can detect temperature and a change in the temperature by measuring the resistance or resistivity of the material which varies with temperature.
The present invention is characterized in that a layer acting as a thermistor is formed on a thin film of diamond responding quickly to heat and that the thin film transfers heat. In particular, a fluid and the thermistor exchange heat with each other via the thin film of diamond.
A second invention of the present application lies in a flow detector using an intrinsic or substantially intrinsic thin film of diamond which has a semiconductor layer of one conductivity type on its surface. The semiconductor layer acts as a thermistor and is formed on the initial crystal growth plane of the thin film of diamond. The final crystal growth plane of the diamond film is in contact with the fluid.
In the second invention described above, the initial crystal growth plane of the diamond film is a plane on which the diamond film begins to grow. As an example, where a thin film of diamond is grown on a substrate, the surface at which the substrate and the thin film of diamond are in contact with each other is the initial crystal growth plane. The final crystal growth plane is opposite in sense to the initial crystal growth plane, i.e., a plane at which the crystal growth ends. In other words, the final crystal growth plane is exposed on the surface after the completion of the film.
As defined in the second invention, a layer acting as a thermistor is formed on one surface of a thin film of diamond, and the other surface is made to touch a fluid whose fluid rate is to be measured. The layer of the thermistor on which conductive interconnects or the like are formed is neither corroded nor deteriorated by the fluid. In one great feature of the invention, diamond which is most resistant to corrosion is used as a protective layer.
A third invention of the present application lies in a flow detector using an intrinsic or substantially intrinsic thin film of diamond which has a layer acting as a thermistor on its surface. This layer is formed on the initial crystal growth plane for the thin film of diamond. The final crystal growth plane of the thin film of diamond is in contact with the fluid.
In the third invention described above, the initial crystal growth plane of the thin film of diamond is a plane on which the diamond film begins to grow. As an example, where a thin film of diamond is grown on a substrate, the plane at which the substrate and the thin film of diamond are in contact with each other is the initial crystal growth plane. The final crystal growth plane is opposite in sense to the initial crystal growth plane, i.e., a surface at which the crystal growth ends. In other words, the final crystal growth plane is exposed on the surface after the completion of the film. This final crystal growth plane is a rough surface which is brought into contact with a fluid, whereby the thermal exchange efficiency can be enhanced. Of course, the roughness of the surface is so small that the flow velocity of the fluid is not affected.
Where the third invention described above is adopted, the advantage of the second invention, i.e., that the fluid is not in direct contact with the layer acting as a thermistor, can be derived. At the same time, the efficiency of thermal exchange with the fluid can be enhanced. This improves the accuracy at which the flow rate can be detected.
A fourth invention of the present application lies in a flow detector using an intrinsic or substantially intrinsic thin film of diamond which has a layer acting as a thermistor on its surface. The thin layer of diamond has a larger heat capacity than that of the layer acting as a thermistor.
The fourth invention is necessary in order that the thin film of diamond acts as a heat storage layer and that a requisite amount of heat is supplied to the thermistor, or a portion at which the layer acting as a thermistor is formed, at a high speed. If the heat capacity of the thin film of diamond is less than that of the thermistor, then the temperature of the thin film of diamond determined by the amount of heat taken away from the thin film by the fluid is affected greatly by the temperature of the thermistor having a larger heat capacity. In consequence, the temperature of the thin diamond film does not correctly reflect the flow rate. That is, the flow rate cannot be precisely measured. Of course, if the heat capacity of the thin diamond film is too large compared with the heat capacity of the thermistor, then the responsiveness of the thin film to heat drops. As a result, the sensitivity with which a flow rate is measured drops.
A fifth invention of the present application lies in a flow detector using a thin film of diamond which comprises plural layers acting as a thermistor together with a heating element. A heat gradient formed across the thin film by the heating element is detected by the plural layers acting as a thermistor.
The thin film of diamond is not uniformly heated by the heat generated by the heating element. In a thermal equilibrium state, (in which transfer of heat exists), temperature differs from location to location on the thin diamond film and also depending on the distance from the heating element and on the flow of the fluid. A heat gradient exists between any two portions at different temperatures. Of course, the state of the heat gradient changes according to variations in the flow rate. Consequently, information regarding the flow rate can be obtained by detecting the heat gradient. A function of detecting the heat gradient consists of detecting the temperatures at different locations on the thin diamond film and calculating the heat gradient from the temperature difference.
A sixth invention of the present application lies in a method of measuring a flow rate by the use of a flow detector comprising a layer and a heating element. The layer is formed on the surface of a thin film of diamond and acts as a thermistor. The heating element is heated by AC waveforms. The output from the layer acting as a thermistor in response to the heating is arithmetically processed to thereby calculate the flow rate of the fluid flowing while in contact with the diamond film.
A seventh invention of the present application lies in a method of detecting the flow rate of a fluid by the use of a flow detector comprising a thin film of diamond, plural layers formed on the thin film and acting as a thermistor, and a heating element. The thin film of diamond is brought into contact with the fluid. A heat gradient created across the diamond film by the heating element is measured. The flow rate is determined from the magnitude of the heat gradient. The direction of the fluid is determined from the direction of the heat gradient. Changes in the flow rate can be known from the manner in which the heat gradient changes.
The flow rate of the fluid is determined from the magnitude of the heat gradient by calculating the flow rate from the magnitude of the heat gradient created according to the flow rate of the fluid. The direction of the fluid is determined from the direction of the heat gradient by utilizing the fact that the direction of the heat gradient is determined by the direction of the fluid. Changes in the flow rate can be known from how the heat gradient changes by calculating a change in the flow rate from a change in the heat gradient reflecting the flow rate.
In the inventions described above, quite great advantages can be had by forming one or more layers acting as a thermistor on one surface of the thin film of diamond and bringing the other surface into contact with the fluid, for the following reasons. Thermal action of the thin diamond film is utilized. At the same time, the structure prevents the fluid from corroding or deteriorating conductive interconnects or the like formed on the layer acting as a thermistor.
The present invention makes use of inactivity, a high thermal conductivity (about 20 W/cmxc2x7deg), and a low specific heat (1.8 J/cm3xc2x7deg) which are features of diamond. Inactivity means that diamond is stable against various corrosive fluids. That is, diamond is resistant to corrosion. In the present invention, a thin film of diamond is doped with boron to control the resistivity value of the diamond film and to impart the function of a thermistor to the instrument. The invention exploits this function of a thermistor.
One example of a fundamental flow detector according to the present invention is shown in FIG. 1, (A)-(C). FIG. 1(B) is a cross section taken along line A-Axe2x80x2 of FIG. 1(A). FIG. 1(C) is a cross section taken along line B-Bxe2x80x2 of FIG. 1(A). A heating element 11, a pair of electrodes 10 for supplying an electrical current to the heating element, a thermistor layer 12 acting as a flowmeter, and a pair of metal electrodes 15 for obtaining an output from the thermistor layer 12 are formed on the thin diamond layer 13. It is necessary that this thermistor layer 12 act as a high-sensitivity thermometer. In FIG. 1(A), the thin diamond film 13 is exposed between the heating element 11 and the thermistor layer 12 because they are separated by a groove as shown in FIG. 1(C). In this structure, the heating element 11 is not in direct contact with the thermistor layer 12. Rather, they are thermally coupled together via the diamond film 13.
The thin diamond film 13 is formed up to a thickness of about 10 xcexcm by CVD. Boron ions are implanted into the surface of the thin film 13 by ion implantation techniques. As a result, a p-type semiconductor layer approximately 0.1 xcexcm thick is formed as the thermistor layer 12. The heating element 11 is also formed by making use of this layer doped with boron ions. In this way, the flow detector shown in FIG. 1, (A)-(C), comprises the thin diamond film 13 acting as a base, a thermistor layer 12, and the heating element 11 to form an integrated structure. The thermistor layer 12 has a p-type semiconductor layer of diamond having a thickness of about one hundredth of the thickness of the thin film 13 on the surface of the thin film 13.
The flow detector can be used in various locations. Therefore, the temperature T0 of the fluid may vary in a short time. As mentioned above, the difference between the temperature T of the flow detector and the temperature T0 of the fluid is important for measurement of the flow rate. If the fluid temperature T0 changes due to a change in the ambient temperature, it follows that the difference Txe2x88x92T0 changes. In this case, if the measured difference Txe2x88x92T0 is very small, or if a change in this difference should be measured, then a large measuring error or a sensitivity change appears. Therefore, a method of detecting flow rates without being affected by ambient temperature variations is needed.
To cope with this problem, the invention is characterized in that a thermistor acting as a flow detector is heated by AC waves. The amount of heat consumed by the thermistor is modulated with frequency w, as given by W exp(iwt). Therefore, Eq. (1) is modified into the following form:
W exp(iwt)=C (xcex94T/xcex94t)+(G+K) (Txe2x88x92T0)xe2x80x83xe2x80x83(9)
where xcex94T/xcex94t is a minute change in the temperature of the thermistor, C is the heat capacity of the thermistor, G is the heat conductivity of the thermistor, and K is a coefficient of heat conduction indicating heat carried away by a fluid. That is, like symbols have like meanings both in Eqs. (8) and (9). In this method using AC heating, T is measured from the output from the thermistor. The coefficient K is found, using Eq. (9). Then, the flow rate is calculated, using a relational formula K=K0{square root over (u)}. xcex94T measured by the flow detector is given by Eq. (10) below
xcex94T=W0/(G+K)(1+w2xcfx842)xe2x80x83xe2x80x83(10)
where T is the above-described time constant defined by xcfx84=C/(G+K).
The operation of the instrument shown in FIG. 1, (A)-(C), is now described, the instrument forming one example of a flow detector according to the present invention. As shown in FIG. 1, (A)-(C), the flow detector comprises the thin film of diamond 13, the thermistor 12 formed on the thin film 13 and using this thin film 13, and the heating element 11. These components 11-13 are integrated in one unit. The thin film 13 acts as the base of the instrument and made of diamond which has a low specific heat and a high heat conductivity. Thus, heat is transferred among the components uniformly and quickly. For example, diamond has a heat conductivity more than 10 times as large as that of silicon.
In the flow detector shown in FIG. 1, (A)-(C), only the thermistor layer 12 acts only as a temperature sensor. The other portions act as good thermal conductors in a sense. More specifically, since the bottom surface 17 is in contact with a fluid 16, heat is carried away by the fluid 16 through the bottom surface 17 which is opposite to the surface of the thin diamond film 13 on which the thermistor layer 12 and the heating element 11 are formed.
In actual operation, heat generated by the heating element 11 is carried away by the fluid 16 at the bottom surface 17 of the thin diamond film 13 which is in contact with the fluid 16. As a result, the temperature T of the thermistor 12 defined by Eq. (8) is determined.
Diamond has a high Debye temperature of 2240 K and a small specific heat of 1.8 J/cm3xc2x7deg. Therefore, the heat capacity of the thermistor layer 12 is quite small. In this case, consequently, high-speed response can be obtained.
Where the thermistor layer 12 acting as a flow detector is integrated with the thin diamond film 13 as shown in FIG. 1, (A)-(C), the diamond film 13 having a thickness (10 xcexcm) 100 times as large as the thickness (0.1 xcexcm) of the thermistor layer 12 and in contact with the thermistor layer 12 acts as a heat storage layer. For example, where the amount of heat K (Txe2x88x92T0) carried away from the thermistor layer by the fluid varies greatly, it has been impossible for the prior art techniques to cope with the resulting variations in the situation. In consequence, Eq. (8) no longer holds. In the structure shown in FIG. 1, (A)-(C), the thin diamond film 13 quickly responds to changes in the heat amount K (Txe2x88x92T0) and supplies heat to the thermistor layer. Consequently, Eq. (8) is satisfied quickly. A situation in which the temperature of the thermistor depends on the flow rate is quickly realized. That is, the diamond film 13 serving as a heat storage layer acts as a quickly responding heat source. As mentioned above, since diamond has a high thermal conductivity (about 20 W/cmxc2x7deg) and a low specific heat (1.8 J/cm3xc2x7deg), heat flows into the thermistor layer 12 from the thin diamond film 13 rapidly.
The heat storage layer consisting of this thin diamond film 13 acts passively and supplies an appropriate amount of heat to the thermistor layer 12 according to the condition. The heat storage layer always acts to correct deviation from the thermal equilibrium state given by Eq. (8). More specifically, the fluid 16 is thermally coupled to the thermistor layer 12 and even to the heating element 11 by the thin diamond layer 13 having a much higher heat conductivity than the fluid 16. When the heating element 11 heats the structure, the thermistor layer 12 is efficiently heated via the thin diamond layer 13 that is a heat storage layer. The diamond layer 13 is about 100 times as thick as the thermistor layer 12 and so the diamond layer 13 has a heat capacity about 100 times as large as the heat capacity of the thermistor layer 12. Therefore, whether the flow rate of the fluid 16 increases slightly or greatly, an amount of heat optimum for realization of a thermal equilibrium state is quickly supplied to the thermistor layer 12 from the diamond layer 13 according to the changing flow rate. A condition satisfying Eq. (8) is accomplished quickly.
In the prior art structure, a heating element is directly used as a heat source for supplying heat to the thermistor. Any component has not existed which supplies heat to the thermistor according to the changing conditions (e.g., temperature variations or flow rate variations of the fluid) to satisfy Eq. (8) at all times, for maintaining a thermal equilibrium state in the thermistor.
When the flow rate drops rapidly, the thermistor layer 12 responds quickly to the decrease in the amount of heat carried away in response to the change in the flow rate because the heat capacity of the thermistor layer 12 is quite small. At this time, the heat storage layer consisting of the thin diamond layer 13 acting passively does not supply excessive heat to the thermistor layer 12. Hence, the thermal equilibrium state is not lost since the heat storage layer operates to maintain the thermal equilibrium state between the thin diamond layer 13 and the thermistor layer 12 which are integral with each other.
In this case, if the amount of heat carried away from the thermistor layer 12 by the fluid decreases, the amount of heat required by the thermistor layer 12 to maintain the thermal equilibrium state decreases. Since the diamond film 13 is thermally coupled to the thermistor layer 12 and acts to maintain the thermal equilibrium state in the thermistor layer 12, the amount of heat just required by the thermistor layer 12 is supplied from the diamond film 13.
As described thus far, the heat storage layer made of the thin diamond layer 13 maintains the thermal equilibrium state, i.e., always satisfies Eq. (8), without being affected by variations in the flow rate. Therefore, in measurement of a flow rate, the temperature of the thermistor layer 12 correctly reflects the temperature of the fluid 16. As a result, the flow rate can be precisely measured.
The operation of each component of the structure shown in FIG. 1, (A)-(C), is described next.
As given by Eq. (8), the sensitivity of the flow detector is proportional to the difference between the temperature T of the thermistor and the temperature T0 of the fluid, i.e., Txe2x88x92T0, We now discuss a situation in which heat is carried off from the surface 17 of the thin diamond film 13 by the fluid 16. Since heat is removed from the surface 17 of the diamond film 13, the surface 17 assumes a temperature approximating the temperature T0 of the fluid 16. The amount of heat removed is supplemented quickly by the heat storage layer, or the diamond film 13 and, therefore, the temperature of the portion of the diamond film 13 in contact with the thermistor 12 does not change greatly. A heat gradient is created across the diamond film 13 as indicated by the arrow 18, i.e., temperature rises in the direction indicated by the arrow. In this way, the difference between the temperature T of the thermistor layer 12 and the temperature T0 of the fluid can be made large. Of course, it is important that the thin diamond film 13 act to satisfy Eq. (8).
Therefore, as already discussed using Eq. (8), the sensitivity of the flow detector can be enhanced substantially. That is, the same effects can be obtained as when the heat conductivity G of Eq. (8) apparently decreased.
In the present invention, the heat capacity of the thermistor itself can be made quite small and so high-speed response can be realized along with the above-described high sensitivity. In particular, a value sufficiently close to 20 W/cmxc2x7K which can be anticipated-from the physical properties of diamond can be obtained as the heat conductivity of the flow detector. At the same time, the heat capacity of the thermistor can be set to a quite small value sufficiently close to 20 xcexcJ/K while taking the size of the thermistor as 1 cmxc3x971 cmxc3x970.1 xcexcm. This quite small value can be anticipated from the physical properties of diamond itself. Consequently, in the time constant xcfx84=C/(G+K) which represents the response speed, the heat conductivity G can be set large, and the heat capacity C can be set small. As a result, 7 can be made small. Hence, high-speed response can be accomplished. If we assume that G=20 Wxc2x7cm/K and C=20 xcexcJ/K, then xcfx84=10xe2x88x926 second.
The present invention is constructed as described thus far. Thus, a high sensitivity which would be obtained when the heat conductance decreased apparently can be derived. At the same time, high response can be obtained. In consequence, these two items which would have been conflicting parameters in the prior art techniques are made compatible with each other.
The operation described thus far is made possible by the structure shown in FIG. 1, (A)-(C). That is, the thermistor layer 12 acting as a flow detector and the heating element 11 are integrally formed on the thin diamond film 13. The heat capacity of the diamond film 13 serving as a heat storage layer is made large compared with the heat capacity of the thermistor layer 12. Therefore, if the heat capacity of the diamond film 13 acting as a heat storage layer is not larger than the heat capacity of the thermistor layer 12, then the diamond film 13 will not act as a heat storage layer responding quickly to temperature variations of the thermistor layer 12. As a result, expected results will not be obtained.
In the description made above, both heating element and thermistor layer are mounted on one surface of the thin diamond film acting as a heat storage layer. The fluid flows along the other surface, or the bottom surface. The same principle can be applied with similar utility to a structure where the fluid flows along the surface on which both heating element and thermistor layer are mounted.
Since a heat gradient is also created in the direction parallel to the plane of the thin diamond film, plural thermistors may be formed so as to detect every heat gradient formed along the diamond film. The flow rate, a change in the flow rate, and the direction of the flow can be determined simultaneously from information regarding the heat gradients. In this case, the flow rate can be computed from the magnitudes of the heat gradients. A change in the flow rate may be calculated from changes in the heat gradients. The direction of the flow can be determined from the directions of the heat gradients.
In the example of FIG. 1, (A)-(C), the surface 17 of the thin diamond film 13 which is in contact with the fluid 16 can be taken as a rough surface in crystal growth using CVD. Since the surface 17 has microscopic irregularities, the surface area can be made substantially large. In this structure, therefore, the fluid 16 carries heat away efficiently from the surface 17. This structure in which the rough surface of the diamond surface is taken as a surface in contact with the fluid is equivalent to increasing the value of K (Txe2x88x92T0) of Eq. (8) which indicates the amount of heat carried away by the fluid. In consequence, the substantial sensitivity can be enhanced in detecting the flow rate.
This makes use of the facts that the surface in contact with the substrate (e.g., a silicon substrate) on which a thin diamond film is grown is smooth and that the opposite surface exposed during the growth of the diamond film has small microscopic irregularities on the order of 100 xc3x85. In this case, the thermistor, the heating element, electrodes, conductive interconnects, and other parts are formed on the smooth surface. This makes it easy to fabricate the instrument.
A measurement can be made without being affected by variations in the ambient temperature by measuring variations in the temperature of the detector portion, or the thermistor, varying according to variations in the heating element by the use of AC driving heating as given by Eq. (2). In particular, heat constantly varying like AC waves is generated by the thermistor so as not to measure variations in the ambient temperature or variations in the temperature of the fluid. Then, xcex94T/xcex94t which is a change in the temperature of the thermistor in a minute time is measured. The amount of heat carried away by the fluid can be determined from the measured value without being affected by the temperature of the fluid or by the ambient temperature. the flowmeter shown in FIG. 1, (A)-(C), a given bias voltage was applied to the resistor 12. The heating element 11 was heated, and the flow rate of fluid, or nitrogen, 16 flowing across the rear surface 17 of the thin diamond film was measured. The results are shown in FIG. 2. The resistances of the resistor 12 and of the heating element 11 were set to about 1 kxcexa9 and about 100 xcexa9, respectively, by adjusting the areas of the resistors. Data listed in FIG. 2 was obtained by making a measurement in which the heating element 11 was driven by AC waves to prevent the heating element from being affected by the ambient temperature. In FIG. 2, the square root of flow velocity (cm/s) is plotted on the horizontal axis. The output from the thermistor was transformed into a voltage and amplified by an amplifier. The output from this amplifier was plotted on the vertical axis.
Let A (cm2) be the cross-sectional area of the fluid. It can be seen from FIG. 17 that the minimum measurable flow rate is less than 1xc3x97A (cm3/s) and the maximum measurable flow rate is about 25xc3x97A (cm3/s). Therefore, the dynamic range has three or more orders of magnitude. Actual measurement has shown that the response speed was on the order of 50 ms. Consequently, it was demonstrated that high-speed response could be obtained.
We consider that the aforementioned large dynamic range, i.e., a large measuring range, arises because the thin diamond film 13 acts as a heat storage layer and thus a thermal equilibrium state according to the flow rate is realized quickly. In other words, the diamond film 13 is operated as a heat sensor having a large dynamic range, and the temperature of this heat sensor is detected by the resistor 12. In addition, this structure functions effectively.
Especially, the heating element 11 and the resistor 12 are mainly thermally coupled to the thin diamond film 13. It is important that most of the heat generated by the heating element flow into the diamond film and that most of heat carried away from the diamond film be carried off by the fluid 16.
These points are essentially different from the flow detector described in the above-cited Sensors and Materials, 2, 6 (1991), pp. 329-346. In the structure described in this reference, a heating element and a sensor portion (which can be taken as a thermally sensitive resistor) are formed in a silicon substrate. The heating element and the sensor portion are thermally coupled to a part of a thin diamond film and also to the silicon substrate.
Since the heating element and the sensor portion are thermally coupled to the silicon substrate, a major part of the heat generated by the heating element also flows into the silicon substrate. As a result, the amount of heat detected by the sensor portion is an amount of heat conducted through the silicon substrate. Therefore, it is impossible that only the amount of heat carried away from the thin diamond film is effectively detected by the sensor portion. Since the sensor portion is formed in the silicon substrate, what is measured by the sensor portion is not the temperature of the diamond film but the temperature of the silicon substrate. Again, it is impossible to precisely evaluate the amount of heat carried away from the diamond film.
Furthermore, the heating efficiency is poor because a large amount of heat escapes from the heating element into the silicon substrate. Hence, this known structure is not practical.
In the structure shown in FIG. 1, (A)-(C), almost all the amount of heat supplied from the heating element 11 is supplied to the thin diamond film 13. The resistor 12 that is also a temperature-sensing resistor is mainly thermally coupled only to the diamond film 13. In consequence, the output from the resistor 12 can correctly reflect the amount of heat carried away by the fluid.
In other words, it is very important to arrange so that the temperature of the diamond thin film 13 is measured by the resistor 12 to measure the flow rate of the fluid flowing in contact with the diamond thin film.
As described above, the mass flow sensor having a large dynamic range may be obtained by disposing the resistor (temperature sensing resistor) and exothermic body so as to be thermally coupled only with the diamond thin film to measure only the temperature of the diamond thin film by the resistor.
However, when a long time measurement of more than several minutes was carried out using the mass flow sensor shown in FIG. 1, a phenomenon was observed in which outputs from the resistor 12 change bit by bit even the flow rate was the same (hereinafter this is referred to as DC drift). When this DC drift exists, a measuring accuracy is lowered even if a measuring sensitivity is high. That is, a reliability of measured values is lowered.
Here the measuring sensitivity indicates that how much the outputs of the resistor vary corresponding to changes of flow rate. The measuring accuracy indicates a reliability of measured values.
This DC drift is considered to be caused by a quantity of heat escaping from the substrate for holding the diamond thin film 13 and wiring leads (gold wires). Although the quantity of heat escaping from the substrate for holding the diamond thin film 13 is small as compare to the quantity of heat taken away by the fluid 16, the component of the DC drift appears in the outputs from the resistor 12 in a measurement of more than several minutes or several tens of minutes. In concrete, the outputs from the resistor 12 drift regardless of the flow rate even though it is a very little.
By the way, though it has been shown from an experiment that the minimum response time of the mass flow sensor shown in FIG. 1 is about 50 msec., a response time of the diamond thin film itself to heat is estimated to be about several msec. This difference is also considered to be caused by the heat escaping to the aforementioned substrate and leads. That is, it may be considered that an influence of the substrate and leads having a longer response time than that of the diamond thin film is appearing in it.
Accordingly, it is an object of the present invention to suppress the fluctuation of measured values due to the DC drift to measure a flow rate in a high reliability.
It is also another important object of the present invention to measure a flow rate in a wide measuring range and in a high measuring accuracy.
The present invention was achieved by noticing on that when a fluid is flown in contact with a diamond thin film, the diamond thin film itself responds thermally to changes of flow rate of the fluid. In concrete, the basic concept is that thermal effects which the thin film material receives corresponding to variations of flow rate of the fluid are quantitively evaluated by measuring response characteristics of the diamond thin film to heat to calculate the quantity of heat taken away from the diamond thin film by the fluid from the response characteristics.
Now the main inventions will be explained. By the way, in the inventions described below, a significant point is that the resistor (temperature sensing resistor) is disposed on the surface of the diamond thin film to detect the temperature of the diamond thin film. Therefore, although it is necessary to consider the problems of sensitivity, stability and productivity, in principle s thermistor generally used or a thermally sensitive resistor such as platinum may be employed. In the description below, a resistor having a function for measuring temperature utilizing that its resistance changes in corresponding to temperature is generally called as a temperature sensing resistor (thermally sensitive resistor). Accordingly, a thermistor shall be included in the category of the temperature sensing resistor.
The eighth invention is characterized in that the resistor having a function for measuring temperature of the diamond thin film is disposed on one surface of the diamond thin film and at least the other surface of the diamond thin film contacts with the fluid.
In such an arrangement, generally a polycrystal diamond thin film formed by CVD is used. Of course, a diamond thin film fabricated by other methods may be used.
Although the resistor may function as an exothermic body, it is basically used for detecting the changes of temperature of the diamond thin film by changes of its resistance. That is, the resistor has to function at least as a temperature sensing resistor. Various thermistors and various metals may be used for the resistor. Basically, any such material whose resistance value changes in corresponding to temperature may be used.
It is important for the resistor to have the function for measuring temperature of the diamond thin film. The resistor must not detect changes of temperature of materials other than the diamond thin film in order to detect the temperature of the diamond thin film accurately by the changes of resistance value of the resistor. Accordingly, the resistor must be arranged so as not to contact with such material that has a high thermal conductivity (such as silicon) other than the diamond thin film. Especially, the resistor must not contact with materials other than the diamond thin film, having a higher heat capacity and higher thermal conductivity than the resistor.
On the other hand, even if the resistor contacts with a material (substance) other than the diamond thin film, an effect of the material to the temperature of the diamond thin film detected by the resistor is negligible if a thermal conductivity of the material is very small or its heat capacity is fully small.
For example, when the resistor is exposed to air, its effect is negligible because the thermal conductivity of air is very small as compare to the thermal conductivity of the diamond thin film. Further, although wiring is provided to the resistor, it will not directly damage the function of the resistor for measuring the temperature of the diamond thin film because a heat capacity of wiring is very small as compare to that of the resistor.
In any case, it must be arranged so that the resistor will not measure temperature of materials other than the diamond thin film. In concrete, it is necessary to arrange so that the resistor contacts only with the diamond thin film.
When a protection film or the like needs to be provided on the surface of the resistor, the condition that the resistor is thermally coupled only with the diamond thin film may be maintained by using a material that meets at least one of the following two conditions:
(1) have a thermal conductivity of less than {fraction (1/100)} of the diamond thin film, and
(2) have a fully small heat capacity as compare to that of the diamond thin film (less than {fraction (1/100)}).
The changes of temperature of the diamond thin film may be detected in high accuracy and heat taken away from the diamond thin film to the fluid may be correctly evaluated by thermally coupling the resistor only with the diamond thin film in such manner.
Further, if the fluid is flown only through the other surface of the diamond thin film (on the side where the resistor is not disposed), the resistor and wires are not exposed to the fluid and thereby the problem of corrosion of the electrodes and wires caused by the fluid may be solved. When the corrosion of the electrodes and wires is out of question, the both surfaces of the diamond thin film may be of course contacted with the fluid.
The ninth invention is characterized in that the resistor is thermally coupled only with the diamond thin film. It is done so to prevent the resistor from sensing changes of temperature of other materials than the diamond thin film as described in the explanation of the aforementioned eighth invention.
As described in the explanations of the eighth and ninth inventions, it is necessary to accurately evaluate the quantity of heat taken away from the diamond thin film by the fluid to accurately measure the flow rate and for that end, it is necessary to realize a circumstance in which the resistor (temperature sensing resistor) is thermally coupled only with the diamond thin film and a thermal coupling of the resistor with other materials is negligible as compare to that with the diamond thin film.
The tenth invention is characterized in that the diamond thin film is held by the substrate while being thermally insulated.
When a quantity of heat is taken away from the diamond thin film by the fluid, temperature of the diamond thin film changes sensitively. A flow rate may be measured by detecting the changes of temperature by the resistor (temperature sensing resistor) provided in contact with the diamond thin film. Accordingly, when a quantity of heat is lost from the diamond thin film to other materials than the fluid, the changes of temperature of the diamond thin film detected by the resistor includes that caused by the quantity of heat escaping to other materials than the fluid.
From a viewpoint of measuring a flow rate, this represents a substantial drop of a sensitivity. It is because, since the quantity of heat escaping from the diamond thin film to other materials than the fluid also changes corresponding to changes of flow rate or the like, the changes of temperature of the diamond thin film will not correspond to the flow rate when the quantity of heat taken away from the diamond thin film to the other materials than the fluid is greater than that taken away from the diamond thin film by the fluid. This problem becomes significant especially when the flow rate finely changes.
Then it is necessary to minimize the quantity of heat escaping from the diamond thin film to other materials than the fluid. Ultimately, it is ideal to float the diamond thin film on which the resistor (comprising the temperature sensing resistor and exothermic body) is disposed totally freely within a fluid. However, means for holding the diamond thin film and wiring to the resistor are indispensable and the loss of heat through them cannot be totally eliminated. However, if the heat lost to others than the diamond thin film is minimized as compare to the heat taken away from the diamond thin film to the fluid, a state in which the diamond thin film is thermally insulated from the substrate may be substantially realized and the quantity of heat taken away by the fluid may be accurately evaluated.
In concrete, the aforementioned goal may be realized by using a teflon or organic resin substrate whose thermal conductivity is very small for the substrate for holding the diamond thin film. For the material having a small thermal conductivity, the influence may be considerably reduced if a material whose thermal conductivity is less than 1 (Wmxe2x88x921Kxe2x88x921) is selected, because a difference between its thermal conductivity and that of the diamond thin film becomes more than 1000 times.
It is also useful to minimize contact points of the diamond thin film and the substrate for holding the diamond thin film to minimize the quantity of heat conducted from the diamond thin film to the substrate.
When a material having a large thermal conductivity such as a silicon substrate is used, the diamond thin film may be substantially thermally insulated from the substrate by oxidizing or nitriding the portion for holding the diamond thin film to reduce its thermal conductivity and by minimizing the contact area with the diamond thin film.
The eleventh invention is characterized in that in the arrangement in which only one surface of the diamond thin film contacts with the fluid and a resistor which functions as a temperature sensing resistor thermally coupled only with the diamond thin film or as an exothermic body is provided on the other surface, the one surface of the diamond thin film contacting with the fluid is the final crystal growth surface of the diamond thin film.
The diamond thin film formed by plasma CVD has a polycrystal diamond structure and a fine irregularity exists on the surface thereof. The surface contacting with the substrate (mainly a silicon substrate is used) has a smooth surface. Accordingly, an efficiency of heat exchange between the fluid may be improved and the quantity of heat taken away by the fluid may be accurately evaluated by arranging so that the final crystal growth surface having the irregularity contacts with the fluid.
Further, because the resistor is disposed on the surface on the opposite side from the final crystal growth surface, i.e. on the initial crystal growth surface which is smooth, the circuit may be readily disposed.
The twelfth invention is characterized in that an exothermic body and thermosensible body are provided on the diamond thin film and the exothermic body and the thermosensible body are thermally coupled only through the diamond thin film.
For the exothermic body, a resistor which generates heat by Joule heat may be used. For the thermosensible body, a temperature sensing resistor whose resistance value changes by temperature may be used. For example, a platinum thin film may be used as the resistor and it may be fabricated discriminately as an exothermic body and a temperature sensing resistor (thermosensible body) by presetting its resistance value discriminately.
In the 12th invention, the significant point is that the exothermic body and the thermosensible body are thermally coupled only through the diamond thin film. A quantity of heat is supplied from the exothermic body to the diamond thin film and a part of the heat is taken away from the diamond thin film by the fluid. Temperature of the diamond thin film is determined corresponding to the quantity of heat taken away by the fluid. Accordingly, the flow rate may be calculated by detecting the temperature of the diamond thin film or the changes of temperature thereof by the thermosensible body.
In this case, the thermosensible body must only detect the temperature of the diamond thin film in order to improve the accuracy of the measurement of the flow rate. In order for that, a quantity of heat reaching to the thermosensible body from the exothermic body via materials other than the diamond thin film must be minimized.
For example, when the exothermic body and thermosensible body contact (thermally coupled) with a material having a high thermal conductivity such as silicon other than the diamond thin film, the temperature sensed by the thermosensible body includes not only the temperature of the diamond thin film, but also that of the material having such high thermal conductivity heated by the heating. Then, in this case, the temperature sensed by the thermosensible body does not reflect the quantity of heat taken away from the diamond thin film by the fluid. That is, an output that accurately reflects the flow rate cannot be obtained from the thermosensible body.
Accordingly, the exothermic body and the thermosensible body must be thermally coupled only through the diamond thin film that contacts with the fluid.
The thirteenth invention is characterized in that the exothermic body and thermosensible body are provided in contact with the diamond thin film, that the exothermic body has a function to heat pulsewise and that the thermosensible body has a function to sense changes of temperature of the diamond thin film caused by the pulsewise heating from the exothermic body.
The flow rate may be measured very accurately by sensing the changes of temperature of the diamond thin film that correspond to the heat pulse (the measurement of the flow rate by means of the heat pulse will be described later in detail). By adopting the aforementioned arrangement, a mass flow sensor may be obtained which allows to accurately measure the flow rate in a larger dynamic range and to measure a mixed ratio of a mixed fluid in which a plurality of fluids are mixed, types of the fluids, concentration of contents within the fluid and others.
The fourteenth invention is characterized in that a diamond semiconductor having one conductive type is provided on one surface of the diamond thin film and that the diamond semiconductor functions as an exothermic body.
In order to detect a flow rate of a fluid flowing in contact with the diamond thin film, it is useful to supply a quantity of heat to the diamond thin film and to sense the temperature of the diamond thin film which corresponds to a quantity of heat taken away from the diamond thin film by a temperature sensing resistor.
In this case, means for supplying the heat to the diamond thin film includes an arrangement in which a current is flown to a resistor to utilize Joule heat generated by the resistor. When the resistor was formed by a platinum thin film formed by vapor deposition on the surface of the diamond thin film for example, it was observed that the diamond thin film moves wavily when the resistor is heated pulsewise by a pulsewise current (intermittent current). It is considered to have been caused by a difference between the thermal expansion coefficient of the diamond thin film and that of the platinum thin film during the instantaneous heating and cooling thereafter.
Because wires are connected to the resistor which functions as the exothermic body and the resistor for sensing temperature is often disposed on the surface of the diamond thin film, there is a high risk of the wires being cut or of the connection parts being disconnected if the diamond thin film moves wavily every time when it is heated pulsewise. Further, noise may be generated in the current flowing through the resistor due to the mechanical vibration.
On the other hand, such wavy motion of the diamond thin film as described above was not observed even when heated pulsewise when B was ion implanted on the surface of the diamond thin film to form a P-type diamond semiconductor layer and to use it as an exothermic body. It is considered to have happened because there is almost no difference between the thermal stress of the exothermic body and that of the diamond thin film and due to that, the diamond thin film would not warp.
The fifteenth invention is characterized in that one surface of the diamond thin film forms a part of the inner wall of a passage for flowing a fluid.
In the above construction, the passage for flowing the fluid may be for example a pipe through which fluid flows. What is important in the above construction is that a mass flow sensor integrated with the passage (e.g. a pipe) may be provided in the passage by constructing the part of the inner wall of the passage through which the fluid flows by the diamond thin film and by disposing the resistor on the surface of the other side of the diamond thin film which does not contact with the fluid. By doing so, a construction which permits to measure the flow rate without disturbing a flow of the fluid may be realized.
Further, as described later, in order to cause the fluid to take away heat effectively from the diamond thin film, it is useful to tilt the diamond thin film more or less against the flow of the fluid so that the fluid readily hits the diamond thin film, not placing the surface of the diamond thin film in parallel to the flow of the fluid. Such arrangement may be included in the arrangement of the fifteenth invention that xe2x80x9cone surface of the diamond thin film forms a part of the inner wall of the passage.xe2x80x9d That is, the flat surface of the diamond thin film needs not to form the totally same surface with the inner wall of the passage necessarily.
The sixteenth invention is characterized in that a passage section in which a mass flow sensor using the diamond thin film is provided is divided into a plurality of passages.
The passage section is divided to reduce Reynolds number at the mass flow sensor section to less than 2000, as described later. Reynolds number is reduced to less than 2000 because a flow becomes unstable and the accuracy of the flow rate measurement drops if Reynolds number is in between 2000 and 4000.
The sixteenth invention presupposes of course that the flow rate measuring range is within the range of Re=2000 to 4000. That is, the present invention is important in that it allows to reduce the maximum value of Re to less than 2000 by adopting the arrangement of the sixteenth invention when Re=2000 to 4000.
The adoption of the structure of the sixteenth invention allows to reduce Rexe2x89xa62000 by the following reason.
Reynolds number Re is represented as Re=dV/v, where the passage is a circular pipe of d in diameter, V is a flow velocity and v is a kinematic viscosity. When the passage is divided here, it means that diameter d is substantially reduced, so that the value of Re may be reduced.
For example, when a circular pipe is adopted for the passage of the fluid, dividing the inside of the pipe into quarters will reduce the sectional area of each of divided passage to a quarter, so that when one divided passage is reformed into a circular pipe, its diameter equals to xc2xd of the pipe undivided. Thereby Re is reduced to xc2xd of that before the division.
Accordingly, if Re=500 to 3000 before the passage is divided in the flow rate measuring range of this case, dividing the inside of the pipe into quarters allows to reduce Re to 250 to 1500 in one divided passage, so that the accuracy of flow rate measurement may be prevented from being lowered due to unstable flows.
Furthermore, the sixteenth invention may be utilized not only in the measurement of flow rate but also in the discrimination of a type of fluid and in checking concentration of contents within the fluid. For example, concentration of contents within the fluid may be measured without being influenced by an unstable flow of Re=2000 to 4000.
The seventeenth invention is characterized in that a fluid which contacts with the mass flow sensor flows nearly in a laminar flow state. According to hydrodynamics, when Reynolds number is less than 2000, a flow turns out to be a laminar flow, when Reynolds number is 2000 to 4000, it is turned into an unstable flow in which turbulent flows are created locally, and when Reynolds number is more than 4000, it turns out to be a perfect turbulent flow which is stable.
That is, a flow is turned into a nearly laminar flow state and a stable measurement can be carried out when Reynolds number of the fluid to be measured in the flow rate measuring range is reduced to less than 2000 by adopting the seventeenth invention. Of course, the measurement in this laminar flow state is useful for other cases than the measurement of the flow rate.
The eighteenth invention is characterized in that Reynolds number of the fluid which contacts with the mass flow sensor is increased to more than 4000. As described above, a stable turbulent flow state may be realized by increasing Reynolds number to more than 4000 and a stable flow rate measurement may be performed by and large.
A sectional area of the passage is narrowed down to increase the flow velocity in order to increase Reynolds number. It is also possible to provide an orifice to choke the fluid and to increase its flow velocity.
However, the increase of Reynolds number to more than 4000 contains such problems as compare to the reduction of Reynolds number to less than 2000 as follows:
(1) the xe2x80x9cstablexe2x80x9d turbulent flow is a matter of degree and a problem that the diamond thin film vibrates due to the turbulent flow still remains, and
(2) the passage of the fluid has to be considerably narrowed down to increase Reynolds number to more than 4000 (in the order of micron for example), which is not realistic structurally.
The nineteenth invention is characterized in that the diamond thin film is disposed at a little angle to a fluid.
FIG. 16 shows an arrangement using this invention. In the figure, a fluid 163 flows in the direction of arrow. A passage 161 for flowing the fluid is an cylindrical pipe in this case. A diamond thin film 162 is placed at an angle of xcfx86 to the flow direction of the fluid.
The fluid can contact with the whole surface of the diamond thin film 162 and the fluid can take away heat effectively from the diamond thin film by placing the diamond thin film 162 at the angle of xcfx86. As a result, the sensitivity in a relatively small flow rate range may be improved. However, this invention has a problem that the diamond thin film disturbs the flow of the fluid.
According to the findings of the inventors, the mass flow sensor described in the eighth invention to nineteenth invention above may be used for the discrimination of a type of fluid, measurement of density of a fluid (quantity of diffusing heat differs depending on density), measurement of mixed ratio of a fluid in which a plurality of fluids are mixed and measurement of concentration of contents within a fluid (e.g. measurement of humidity). That is, the mass flow sensor can be used for measurements utilizing that a quantity of heat taken away from the diamond thin film differs depending on; 1) a type of a fluid, 2) density of a fluid, 3) a mixed ratio of a fluid, and 4) a mixture contained in a fluid.
The 20th invention is characterized in that a predetermined quantity of heat is supplied to a thin film material and a flow rate of a fluid flowing in contact with the thin film material is calculated from changes of temperature of the thin film material caused by the heat.
The 21st invention is characterized in that an operating method of a mass flow sensor in which a resistor is provided on one surface of a thin film material has the following steps.
First step: Outputs from the resistor are integrated in time xcex94t0 to find an integrated value S0.
Second step: A specified quantity of heat is supplied to the thin film material. This step is carried out for example by heat pulse.
Third step: The outputs from the resistor which change corresponding to the second step above are integrated in time xcex94t2 to find an integrated value S2.
Fourth step: A difference between the integrated values S1 and S2 are found.
Fifth step: The flow rate of the fluid is calculated from the result of the fourth step.
In the first step, the outputs from the resistor which is a temperature sensing resistor are integrated in a predetermined time to determine a criterion value for measuring the flow rate. The outputs from the resistor fluctuate, though it is very small. That is, the outputs from the resistor contain noise. Then, the outputs from the resistor are integrated in time xcex94t0 cancel out this fluctuation. By doing so, the outputs that fluctuate in the directions of plus and minus are canceled out and the criterion value which is not effected by the fluctuation of the outputs from the resistor may be determined.
The heating carried out in the second step is preferable to be carried out in a short time pulsewise and is generally carried out from an exothermic body provided on the diamond thin film. As a result of the heating, the diamond thin film is heated up in a very short time and the outputs from the resistor change. The changing state of the outputs correspond to the quantity of heat taken away from the thin diamond film. That is, the changing state of the outputs correspond to the flow rate of the fluid.
In the third step, transient response characteristics of the diamond thin film to the heating may be quantitively evaluated by integrating the changing outputs from the resistor. Although the third step is generally carried out in both states in which the diamond thin film is heated up and is cooled down after finishing the heating, it may be carried out only in either state.
Then in the fourth step only the changes of temperature of the diamond thin film that correspond to the heating may be evaluated by finding the difference between the integrated value obtained in the second step and that obtained in the third step. This changes of temperature of the diamond thin film correspond to the flow rate and contain almost no DC drift component, so that the flow rate value which is not effected by the drift component may be calculated from the result of the fourth step in the fifth step.
Although the measurement of the flow rate is carried out through the series of the steps above, because the one cycle of steps from the first to fifth steps is carried out in succession, the measurement of the flow rate is carried out at regular intervals. This flow rate measuring interval (this time is denoted as T here) must not be effected by the heating of the previous step. That is, it is necessary to enter the next series of steps after the changes of temperature of the diamond thin film which correspond to the heating of the diamond thin film in the second step settle down. This interval T is preferable to be more than 10 times of the heating time (xcex94t0) and it has been confirmed experimentally that when xcex94t0=0.2 seconds, it is necessary to set T=2 seconds or more to operate stably.
Further, it is very useful to use the diamond thin film for the thin film material as described later.
Referring now to the mass flow sensor shown in FIG. 1, the basic principle of operation of the 21st invention will be explained. In this section, a case when the diamond thin film from which remarkable results have been obtained experimentally is used will be mainly explained.
In the mass flow sensor shown in FIG. 1, the exothermic body 11 made of a platinum thin film and the resistor 12 which functions as a temperature sensing resistor and is made of a platinum thin film similarly are provided on one surface of the diamond thin film 13 and the fluid 16 flows in contact with the other surface 17 of the diamond thin film 13.
In the construction shown in FIG. 1, when heat pulse is applied from the exothermic body 11, the heat propagates through the diamond thin film in high speed. This propagation in the diamond thin film is the fastest among that in existing materials. Although the heat conducted from the exothermic body 11 escapes from the back surface 17 and sides of the diamond thin film 13, much of it also reaches to the resistor 12 which is a temperature sensing resistor. The resistor 12 detects the heat conducted as such as the temperature of the diamond thin film 13. This state is thermally an unequilibrium state, and not an equilibrium state. That is, the resistor 12 detects the transient response of the diamond thin film 13 to the heating as the changes of temperature of the diamond thin film.
On the other hand, the fluid 16 flows at the back surface 17 of the diamond thin film 13, and a certain amount of heat is taken away from the diamond thin film by the fluid. The quantity of heat taken away by the fluid depends on the type of the fluid and on the flow velocity (i.e. flow rate). That is, when the flow rate of the fluid 16 changes, the quantity of heat taken away from the back surface 17 of the diamond thin film 13 also changes. This change also influences the quantity of heat arriving to the resistor 12, of course. Accordingly, the transient response characteristics of the diamond thin film 13 to the heating change by the changes of the flow rate of the fluid 16.
That is, though much of the heat conducted to the back surface 17 is reflected by the surface, the reflection rate and how it is reflected depend on the quantity of heat taken away from the back surface 17 by the fluid 16. Then much of the heat conducting within the diamond thin film 13 is reflected by the back surface 17, so that the heat conducted within the diamond thin film 13 is largely influenced by the difference of the flow rate. This influence is effected in very high speed and in high sensitivity by the thermal responsibility which the diamond itself has.
As a result, the movement of the heat pulse supplied from the exothermic body 12 within the diamond thin film (i.e. thermal conductivity) is largely influenced by the flow rate of the fluid 16 which contacts with the back surface 17 and this influence is output as changes of resistance value of the resistor 12 reflecting the changes of temperature of the diamond thin film 13. Then, the flow rate of the fluid 16 may be calculated by processing the outputs from the resistor 12.
Considering thermal influences which the diamond thin film 13 receives from the environment, it is understood that most of them come from the front and back surfaces thereof. It is apparent considering the feature of the thin film. That is, in a case of FIG. 1 for example, although the thickness of the film is shown to be thick, the thickness is actually around several microns to several tens of microns while the size thereof is in the order of mm or more.
It is also understood that in the construction shown in FIG. 1, because the exothermic body 11 and the resistor 12 (temperature sensing resistor) are provided on one surface covering the most of the diamond thin film 13 and the fluid 16 contacts only with the back surface 17 to which the diamond thin film 13 is exposed, the thermal influences which the diamond thin film receives from the outside come mostly from the back surface 17.
Then, it is understood that the influence which the heat moving within the diamond thin film 13 receives comes from the back surface 17 which contacts with the fluid 16. Considering as such, it may be considered that how the heat pulse supplied from the exothermic body 11 is conducted within the diamond thin film or the transient response characteristics of the diamond thin film 13 is determined by the flow rate of the fluid 16.
It is then understood that information on the flow rate of the fluid 16 may be obtained by heating pulsewise the diamond thin film 13 by the exothermic body 11 and by measuring the transient response characteristics of the diamond thin film 13 at that time. In concrete, the flow rate of the fluid 16 may be calculated by processing the changes of temperature of the diamond thin film 13 by the procedure shown in the 21th invention.
Referring now to FIG. 6A, the principle of the measurement will be explained more concretely. Here, the fluid is assumed to be flowing in a constant flow rate. Assume also that in FIG. 6A, heat is applied from the exothermic body 11 during a time from t1 to t1+xcex94t1. The time xcex94t1 is about 0.2 seconds for example. As a result, the outputs from the resistor 12 change as indicated by a curve 61. That is, because the diamond thin film is gradually heated up when a certain quantity of heat is supplied from the exothermic body 11, the output f(V) thereof changes as indicated by the curve 61. The changes of f(V) indicate the transient responses of the diamond thin film 13 to the heat applied.
As it is apparent from the discussion above, the flow rate of the fluid 16 may be found by quantitively evaluating the transient response characteristics of the diamond thin film 13 to the heat applied, and the transient response characteristics of the diamond thin film 13 is evaluated quantitively by quantitively evaluating the curve 61 which f(V) draws in this case. In concrete, the transient response characteristics of the diamond thin film 13 is quantitively evaluated by finding an area of a figure drawn by the curve 61.
Because some quantity of heat is taken away by the fluid 16 even after the heating from the exothermic body 11 has ended (after t1+xcex94t1), the output f(V) from the resistor 12 approaches to the original value f0 drawing a curve as indicated by the curve 61 in FIG. 6A. That is, even after the supply of the heat has been finished, the transient response characteristics of the diamond thin film may be evaluated by looking into how the diamond thin film is cooled down.
Thus the response characteristics of the diamond thin film to the heat may be evaluated by quantitively finding how the diamond thin film is heated up and how it is cooled down.
While the curve 61 indicates the changes of the output f(V) from the temperature sensing resistor which corresponds to a certain flow rate, the output f(V) draws such a different curve as indicated by a curve 62 when the flow rate is zero or is small. It is because the diamond thin film is heated up rapidly and is cooled down slowly since less quantity of heat is taken away from the diamond thin film.
Referring now to the following equations, the response characteristics of the diamond thin film to the heat pulse (intermittent heating) will be explained. In the following explanation, W0 denotes a quantity of heat supplied to the diamond thin film by the pulsewise heating in xcex94t1, K a coefficient of thermal conductivity related to a quantity of heat taken away by the fluid, C a thermal capacity of the diamond thin film and G a thermal conductivity of the diamond thin film.
At first, when the parameter of time t meets (t1 less than t less than t1+xcex94t1) or during when the heating by means of heat pulse is carried out, Equation (1) below is satisfied. By the way, when xcex94T/xcex94t=0 (i.e., constant temperature heating) in Equation (1), it turns out to be Equation (8).
W0=Cxcex94T/xcex94t+(G+K) (Txe2x88x92T0)xe2x80x83xe2x80x83(1)
In Equation (1) above, T denotes the temperature of the diamond thin film and T0 denotes the temperature of the fluid. Equation (1) above is a differential equation in which the movement of heat in the diamond thin film is considered in a state when the diamond thin film is heated up.
Further, when the parameter of time t meets (t1+xcex94t1 less than t less than T) or in a state when the heating by the heat pulse has been finished and the diamond thin film is being cooled down, Equation (2) below is satisfied. T denotes a time at which one time of flow rate measurement is finished.
0=Cxcex94T/xcex94t+(G+K) (Txe2x88x92T0)xe2x80x83xe2x80x83(2)
Equation (2) above is a differential equation in which the movement of heat in the diamond thin film is considered in the state when the diamond thin film is cooled down after the heating.
Equation (3) below may be found by solving the above differential equations in (t1 less than t less than t1+xcex94t1).
xcex94T=W0(1xe2x88x92exp(xe2x88x92xcex1t))/(G+K)xe2x80x83xe2x80x83(3)
xe2x80x83xcex1=(G+K)/C
Further, Equation (4) below may be found in (t1+xcex94t1 less than t less than T).
xcex94T=W0exp(xe2x88x92xcex1t)/(G+K)xe2x80x83xe2x80x83(4)
xcex1=(G+K)/C
xcex94T denotes the changes of temperature of the diamond thin film in Equations (3) and (4) above. The above results show the states when the diamond thin film is heated up rapidly and when it is cooled down rapidly as shown in FIG. 6B.
The heating of the diamond thin film shown in FIG. 6B may be carried out in a short time by increasing the value of xcex1. Accordingly, it is useful to use a diamond thin film having a small capacity C and a large thermal conductivity G.
In the discussion above, it is presupposed that the value of f0 is determined by the flow rate. That is, it is presupposed that the value of f0 will not change if the flow rate does not change even in a prolonged flow rate measurement. However, the value of f0 fluctuates in actual operations. This is caused by that the temperature of the diamond thin film 13 gradually changes. The value of f0 fluctuates also by changes of temperature of operating environment (e.g. changes of temperature of the fluid).
This fluctuating component (DC drift component) may be removed by measuring only the transient response characteristics of the diamond thin film to the heat applied. That is, only a variation how the temperature of the diamond thin film changes by the instantaneous heat applied in xcex94t1 should be measured. At this time, although the DC drift component also changes slightly, it is negligible because the changes of temperature of the diamond thin film in the time xcex94t1 is minor.
In concrete, only the variation of f(V) that corresponds to the heat applied may be obtained by integrating the values of f0 just before the heating from the exothermic body 11 for a certain period of time, by integrating the values of f(V) during the heating immediately after that and during cooling also for a certain period of time and by finding a difference between them. The variation of f(V) is found as an amount of integration (integrated value) represented by the hatched portion in FIG. 6A. The variation of f(V) corresponds mainly to the quantity of heat taken away from the surface of the diamond thin film by the fluid and contains almost no DC drift component. The DC drift component questioned here is only the DC drift component generated during the time from immediately before the heating (i.e. t1) to the end of the heating, i.e. t1+xcex94t1, which causes almost no problem.
The measurement above is especially characterized in that only the variation of f(V) caused by the instantaneous heat applied in xcex94t1 is evaluated.
As described above, the flow rate of the fluid flowing in contact with the surface of the diamond thin film may be accurately measured by heating the diamond thin film instantaneously and by evaluating its response characteristics to the heat applied, or how the diamond thin film is heated up and how it is cooled down.
A responsibility of a thin film material to heat is evaluated by xcfx84=Cxcfx81L2/Kxcfx802. This equation is found by neglecting a thermal conductivity in the direction of film thickness and by considering a thermal conductivity in the plane direction by employing a 2-D model. Here, xcfx84 is a parameter indicating a time which the thin film takes for reaching to an equilibrium state from when heat has been added to the thin film. xcfx84 is one of criterion and can be utilized for relative evaluation of values of response time of the thin film to heat, though it does not allow to evaluate the absolute value of the response time.
In the above equation, C denotes a thermal capacity, xcfx81 a density, L a dimension of the thin film (here the thin film is assumed to be square and one side thereof is taken), K a thermal conductivity, and xcfx80 a ratio of the circumference of a circle to its diameter. When xcfx84 of a 4 mm square diamond thin film was evaluated using the equation, it was about 2.2 ms. Also when xcfx84 of a 4 mm square monocrystal silicon thin film was evaluated similarly, it was about 19 ms. It can be concluded from this result that the response time of the diamond thin film to heat is greater than that of the silicon thin film by more than about 8 times. Thus the usefulness of using the diamond thin film in the arrangement of the present invention in which the transient response characteristics due to the heat pulse is evaluated is understood.
Especially a diamond thin film formed by a vapor phase method has a polycrystal state and a crystal structure in which crystal has grown in the direction of film thickness, so that the thermal conductivity in the film thickness direction is expected to be near 2000 (Wmxe2x88x921kxe2x88x921). Accordingly, the arrangement in which one surface of the diamond thin film contacts with the fluid and the exothermic body and thermosensible body are disposed on the other surface is what utilizes the thermal effect very effectively.
For example, because a thermal conductivity of a monocrystal silicon is about 150 (Wmxe2x88x921kxe2x88x921) (300 K), a response time is delayed more than 10 times as compare to the case when the diamond thin film is used, in an arrangement in which one surface of the silicon thin film contacts with the fluid and the exothermic body and thermistor are disposed on the other surface. That is, the time necessary for one cycle of steps shown in FIG. 6 is required to be more than 10 times of the case when the diamond thin film is used.
Further, when the silicon thin film is used, it is influenced more by the quantity of heat escaping to others than the fluid in proportional to its low thermal conductivity, so that a measurement accuracy and dynamic range for a small flow rate is lowered remarkably.
As described above, it is very useful to use the diamond thin film for measuring flow rates from the transient response characteristics which corresponds well to heat.
Now a case when the measuring method shown in FIG. 6 is used using thin film materials other than the diamond thin film will be considered.
When the measuring method shown in FIG. 6 is adopted, the thin film is required to respond.to rapid heating to improve a sensitivity for detecting flow rates.
The response time may be evaluated by Expression Cxcfx81L2/Kxcfx802 described above. If the response time of the thin film to heat applied is short, the time xcex94t1 in FIG. 6 may be shortened.
Also as seen from Equation 3 or 4, it is useful to increase the value of xcex1 to increase the response speed. In the same time, the large value of a means that the sensitivity is high so much. It is because the large value of xcex1 means that changes of output from the temperature sensing resistor indicated by the curve 61 in FIG. 6A are sharp and it means that the outputs of the temperature sensing resistor largely change corresponding to slight changes of flow rate.
As shown by xcex1=((G+K)/C), the greater the thermal conductivity G of the thin film material and the smaller the thermal capacity C thereof, the greater a becomes.
From the above consideration, it is concluded that when a material having a small thermal conductivity and large thermal capacity is used;
1) the values of xcex94t1 and xcex94t2 in FIG. 6 have to be increased because the response time to heat is prolonged (the response speed becomes slow because a becomes small), and
2) the dynamic range of the flow rate measurement is narrowed down because the range of response to heating and cooling is narrow.
However, it is also possible to use other materials than the diamond thin film depending on a flow rate measuring range and measuring sensitivity required. For example, when a monocrystal silicon thin film is used instead of the diamond thin film, although the flow rate measuring accuracy and flow rate measuring range are lowered to less than several tenth as compare to the case when the diamond thin film is used, it is useful to use the silicon thin film from the aspects of productivity and economy when a required flow rate measuring range is 10 slm to 20 slm or when the measuring interval can be several ten seconds.
For the materials other than the monocrystal silicon, silicon carbide, silicon oxide, silicon nitride, mica and silicon in which B or P is doped may be used. Further, their crystal structure may be selected considering characteristics required. Aluminum, aluminum oxide, aluminum nitride, sapphire or the like may be also used. Further, organic resin materials and industrial plastic materials may be used if their low characteristics are permitted.
However, it is desirable to use a material whose thermal conductivity is large and whose thermal capacity is small in order to obtain a high measuring accuracy and large measuring range. Although it is conceivable to use various metallic materials as a material that meets such conditions, the part which contacts with the temperature sensing resistor has to be electrically insulated in such a case.
Further, a thickness of those materials is required to be less than several ten microns. It is because a difference of response speeds caused by a difference of physical properties of materials becomes significant unless a thermal conductivity in the thickness direction of the thin film material is negligible. Generally, the thickness is preferred to be less, than {fraction (1/100)} of size of the film. For example, a thickness of a 4 mm square thin film is preferred to be less than 40 micron. Of course, the thinner the thickness of the thin film material, the better it is.
Moreover, the thermal capacity of the temperature sensing resistor section is preferable to be small. It is because if a thermal capacity of the thermistor section is large, the response time of the temperature sensing resistor itself to heat is prolonged, thereby lowering the sensing accuracy of the thermal response characteristics of the thin film material to heat pulse. Further, it is because it disallows to sense small changes in the quantity of heat.
The inventors verified the operation by the heat pulse as shown in FIG. 6 by using mica thin film of 5 mm square and 10 micron thick. However, its sensitivity, measuring accuracy and dynamic range were remarkably low as compare to the case when the diamond thin film was used and it was just in a level in which the operation could be confirmed.
As described above, by providing the temperature sensing resistor and exothermic body on one surface of the diamond thin film and contacting the fluid with the other surface, 1) the flow rate of the fluid in contact with the diamond thin film may be found from the outputs of the temperature sensing resistor, and 2) an arrangement may be made in which the temperature sensing resistor, exothermic body and wires are not exposed to the fluid.
Further, in the above arrangement, the flow rate may be measured by supplying a predetermined quantity of heat to the diamond thin film and by evaluating the transient response characteristics at that time from the outputs of the temperature sensing resistor. The DC drift component contained in the outputs may be removed and the flow rate can be accurately found by finding the difference between an integrated value of the outputs from the temperature sensing resistor before the predetermined quantity of heat is supplied and that after it is supplied.
As described above, according to a basic operation method of the present invention, heat is applied intermittently or pulsewise to a thin film material having a high thermal conductivity, and a thermal response characteristics of the thin film material caused by the heat application is measured to evaluate a state in which a fluid in contact with the thin film material takes away heat from the thin film material. The case in which the transient response characteristics shown by the thin film material (in this case a diamond thin film) as heat is applied to it in pulse form are to be evaluated for different fluids (stationary fluids) will now be considered. Because different fluids generally have different thermal conductivities and specific heats, different fluids remove heat from the thin film material in different quantities and at different rates. Therefore, for a given condition of the pulse-form heat supply to it, the heating and cooling of the thin film material, i.e. its transient response characteristics, are different for different fluids. So, because differences between different fluids are reflected in differences in these transient response characteristics, different fluids can be distinguished from each other.
Next, the case in which a solid is put in contact with the thin film material, instead of a fluid, and pulse heating is carried out, will be considered. In this case also, for a given state of the pulse-form heat supply to it, the transient response characteristics manifest by the thin film material will differ according to the thermal conductivity and specific heat of the solid material, and from this it follows that differences in thermal conductivity and specific heat between different solids can be detected and measured by this means.
Furthermore, because the density of a solid and the amount of impurities present in it also affect the thermal conductivity and specific heat of the solid, it is possible to detect and measure differences in these quantities also.
As explained above, by applying heat in pulse form to a thin film material of high thermal conductivity and evaluating the thermal response characteristics displayed by the thin film material as the pulse-form heat is supplied to it, it is possible to detect and measure differences in the thermal conductivity and specific heat of different substances (irrespective of whether the substances are gases, liquids or solids) brought into contact with the thin film material.
Materials having high thermal conductivities and low specific heats can be used for the thin film material in this invention. Materials having thermal conductivities equal to or very close to that of diamond are particularly suitable.
Materials having thermal conductivities relatively close to that of diamond thin film include SiC (silicon carbide), cBN (cubic BN), A1N, BeO and BP. When materials like these are used, compared to cases in which Si is used, characteristics relatively close to those obtained when diamond thin film is used can be expected.
Also, if the ambient temperature is a low temperature of less than 100 K, thermal conductivities of near to or over 1000 (W/mK) can be expected of even Si and SiC, and therefore these materials also can be used instead of diamond thin film.